1. Field of the Invention
The present invention relates to a direct-heated flow measuring apparatus having a film resistor which serves as a temperature detecting means as well as an electric heater. Such a direct-heated flow measuring apparatus can be used, for example, for measuring the flow rate of engine intake air.
2. Description of the Related Art
Generally, in an internal combustion engine, the amount of intake air is one of the most important parameters for controlling the fuel injection amount, ignition timing, and the like. A gas-flow measuring apparatus, i.e., an airflow meter, is provided for measuring the same. One of the more common prior art airflow meters is the vane-type, which is, however, disadvantageous in scale, response speed characteristics, and the like. Recently, airflow meters having temperature-dependent resistors have been developed, which are advantageous in scale, response speed characteristics, and the like.
There are two types of airflow meters having temperature-dependent resistors, i.e., the heater-type and direct-heated type. The heater-type airflow meter may consist of an electric heater resistor provided in an intake-air passage of an engine and two temperature-dependent resistors arranged on the upstream and downstream sides of the electric heater resistor. In this case, the temperature-dependent resistor on the downstream side is used for detecting the temperature of air heated by the heater resistor, while the temperature-dependent resistor (temperature-compensating resistor) on the upstream side is used for detecting the temperature of non-heated air. The current flowing through the heater resistor is controlled for a constant difference in temperature between the two temperature-dependent resistors, thereby detecting the voltage applied to the heater resistor as the mass flow rate of air.
In this heater-type airflow meter, if no temperature-compensating resistor upstream is provided and the current of the heater resistor is controlled for a constant temperature of the downstream temperature-dependent resistor, the voltage applied to the heater resistor is detected as the volume flow rate of air.
On the other hand, the direct-heated type airflow meter may consist of a film resistor which serves not only as an electric heater, but also as a temperature-detecting means for detecting the temperature of the heated air. Also, the direct-heated type airflow meter may consist of a temperature-dependent resistor (temperature-compensating resistor) for detecting the temperature of non-heated air. Thus, the current flowing through the film resistor is controlled for a constant difference in temperature between the film resistor and the temperature-compensating resistor, thereby detecting the voltage applied to the film resistor as the mass flow rate of air. In this direct-heated type airflow meter, too, if no temperature-compensating resistor is provided and the current of the heater resistor is controlled for a constant temperature of the film resistor, the voltage applied to the film resistor is detected as the volume flow rate of air.
Since the film resistor of the direct-heated type airflow meter serves as a temperature-detecting means for heated air, that is, no additional temperature detecting means for heated air is necessary, the direct-heated type airflow meter is smaller in size than the heater-type airflow meter.
In the direct-heated type airflow meter, the film resistor may consist of an insulating substrate such as a ceramic substrate or monocrystalline silicon substrate, a film resistance pattern of platinum (Pt), gold (Au), etc. on the insulating substrate, and a heat-resistant resin on the resistance pattern.
Usually, the response characteristics and dynamic range of the direct-heated type airflow meter are dependent upon the heat mass and adiabatic efficiency of the film resistance pattern, which serves not only as a heating means but also as a temperature detecting means. In order to obtain the most excellent response characteristics and largest dynamic range, the film resistance pattern should be ideally in a completely floating state in the air stream. In the prior art, however, the film resistor including the film resistance pattern has had an approximately definite width over the lengthwise direction thereof. Accordingly, the adiabatic efficiency is relatively low, thus reducing the response characteristics and dynamic range of the heat-directed airflow meter.
To alleviate this problem, a direct-heated type airflow meter may be suggested in which an aperture is provided between the heating and temperature detecting portion of the substrate including the film resistor and its supporting member of the substrate, thereby creating a throttling effect on the heat transfer, and thus increasing the adiabatic efficiency of the heating and temperature detecting portion and improving the response speed and dynamic range of the airflow meter.
Note that, usually, the heat transfer throttling portion has a small cross-section in order to obtain a further adiabatic efficiency.
However, even if such a heat transfer throttling portion is provided, some heat is still transmitted to the supporting member and as a result, it takes a long time for the heat transmitted to the supporting member, such as a ceramic having bad dissipation characteristics, to become stable, which means that the airflow meter has a bad response characteristic. Also, since the connections from the substrate to a stay within a duct are usually conventionally carried out by lead terminals (pins), the heat of the heating and temperature-detecting portion of the substrate is transmitted via the lead terminals to the duct. In other words, the adiabatic effect of the substrate is small, and accordingly, the heat loss is large, thereby also deteriorating the response characteristics of the airflow meter.
Further, in the conventional direct-heated airflow sensor for detecting the mass flow rate of air, the film resistance pattern as the heater and temperature-detecting portion and the temperature-compensating resistor are disposed at quite different positions. For example, the film resistance pattern is provided within the duct, and the temperature-compensating resistor is provided outside of the duct. Therefore, due to the difference in heat capacity between the film resistance pattern including its supporting system and the temperature-compensating resistor including its supporting system, the transient temperature characteristics of the system of the film resistance pattern are different from those of the system of the temperature-compensating resistor. As a result, the difference in temperature between the film resistance pattern and the temperature-compensating resistor during a transient state is fluctuated, thereby generating an error in the measured flow rate of air.